The present invention relates to an optical receiver, and particularly to an optical receiver in which a flexible printed circuit board is used for transmission lines between a pre-amplifier and a post-amplifier.
An optical receiver module in which a semiconductor photodiode is used as a receiver element is one of the key devices of transceivers for optical fiber transmission. With the recent spread of broadband networks, optical transceivers have increased in speed and the optical transceivers with a bit rate of up to 10 Gbits/s are beginning to be widely used. The optical receiver modules suitable for the above-mentioned application are required to be downsized and manufactured at low cost.
JP-A No. 2005-217284 discloses an optical transceiver in which an optical receiver sub-assembly is downsized and can be manufactured at low cost by incorporating a semiconductor photodiode and a pre-amplifier in a CAN-type package. In the optical transceiver, connecting of the optical receiver sub-assembly to a main printed circuit board through a flexible printed circuit board allows for absorption of stress caused by dimensional deviation or by deformation due to temperature fluctuations, thereby preventing a stress-generated defect.
However, in the optical receiver disclosed in JP-A No. 2005-217284, an in-band gain reduction occurs while an electrical signal output from the optical receiver sub-assembly reaches a post-amplifier on the main printed circuit board. As a result, there is a problem that SRS (Stressed Receiver Sensitivity) which is one of the evaluation indexes of receiver sensitivity of optical transceivers for 10 Gbits/s Ether is deteriorated. Here, SRS is an indicator to evaluate whether an optical signal after transmission is properly received under the condition that a transmission electrical signal is superimposed with noise in advance. In order to improve SRS, it is preferable to make a small signal gain at around 5 GHz larger than a small signal gain at DC by about 0.5 dB.
“ROSA for 10 Gbps Optical Transmission” of Eudyna Devices Inc. shows a 10 Gbits/s optical receiver sub-assembly including a flexible printed circuit board for connecting a main printed circuit board.
The flexible printed circuit board used in the prior art employs a thin board in order to obtain flexibility. There is ordinarily used the flexible printed circuit board formed by applying a copper foil having an about 30 μm thickness on each of both faces of a polyimide membrane having an about 50 μm thickness. In the case of forming transmission lines with a characteristic impedance of 50 Ω in a microstrip format on the flexible printed circuit board, each of the line widths needs to be extremely thin, like about 80 μm. Therefore, a transmission loss on the flexible printed circuit board becomes a considerable amount. The calculation of the amount of transmission loss in the transmission lines shows that 0.12 dB/cm is for dielectric loss at a frequency of 10 GHz and 0.28 dB/cm is for conductor loss at a frequency of 10 GHz, so that a transmission loss of 0.40 dB/cm is expected in total. The transmission loss increases substantially proportional to a frequency. For example, in the case where the flexible printed circuit board 2 cm in length is connected to the optical receiver sub-assembly, in-band gain reduction from DC to 5 GHz is increased by 0.4 dB throughout the flexible printed circuit board. In addition to this, bandwidth deterioration is increased at discontinuous parts in a connected portion between the flexible printed circuit board and the main printed circuit board, and a connected portion between the flexible printed circuit board and the optical receiver sub-assembly.
A pre-amplifier IC such as the optical receiver sub-assembly incorporated in the optical receiver module is generally designed such that the in-band characteristic up to an IC output portion is made flat. As a result, in-band gain reduction of at least 0.4 dB or more normally occurs from DC to 5 GHz in an input portion of the post pre-amplifier on the main printed circuit board.
As a method of compensating the gain reduction, there is devised means for altering the flat in-band characteristic up to the output portion of the pre-amplifier IC into the in-band characteristic in which the gain is increased according to a frequency by customizing mounting conditions within the optical receiver sub-assembly. According to a study by the inventors, making longer a grounding wire of an input circuit portion of a pre-amplifier, or making longer a connection wire between a pre-amplifier and an optical receiver element (APD or PIN-PD) or between an optical receiver element and an RF ground capacitor allows for boost of the frequency characteristic, thereby obtaining a desired characteristic from DC to 5 GHz.
However, departing from the normal operation of the pre-amplifier, the pre-amplifier is operated such that gain peaking is applied at a high frequency and as a result, the instability of the circuit is enhanced. Consequently, when voltage and temperature are changed within a range of conditions necessary for guarantee of proper operations, there is a case that a defect such as oscillation of the optical receiver sub-assembly occurs under the conditions of high power voltage and low operation temperature. Further, even in the case where no oscillation occurs, the unnecessary gain peak is increased at a high frequency, which causes a problem in the reception characteristics of the optical transceiver in many cases.
As described above, for the stability of manufacturing the optical transceiver, it has been desired that the pre-amplifier is operated in a state where no unreasonable peaking is applied within the optical receiver sub-assembly and that the in-band gain reduction on the flexible printed circuit board is compensated after output from the optical receiver sub-assembly.